One of the most important of the primary fossil fuels, a dark-colored, carbon-rich material that occurs in stratified, sedimentary deposits. Two major periods of coal formation are known in geologic history. The older includes the Carboniferous and Permian periods (from about 350,000,000-250,000,000 years ago). Much of the bituminous coal of eastern North America and Europe is Carboniferous in age. Most coals in Siberia, eastern Asia, and Australia are of Permian origin. The younger era began in the Cretaceous Period (about 135,000,000 years ago) and culminated during the Tertiary Period (about 65,000,000-2,500,000 years ago). From this era came nearly all of the world's lignites and subbituminous (brown) coals.